Abstract
The effects of exposed pipe encasements on the local variation of hydrodynamic and sediment conditions in a river channel are examined. Laboratory experiments are performed to assess the response of water level, flow regime and bed deformation to several representative types of concrete encasements. The experimental conditions considered are: three types of exposed pipe encasements exposed on the bed, including trapezoidal shape, circular-arc shape and polygonal shape, and three sets of discharges, including annual discharge, once-in-3-year flood, and once-in-50-year flood. Our experiments show that: (1) the amount of backwater definitely depends on the encasement geometric shape and the background discharge; (2) smaller discharges generally tend to induce local scour of river bed downstream of the encasement, and the order of sensitivity of bed deformation to the encasement geometric shape is trapezoidal > circular-arc > polygonal; (3) comparatively speaking, the polygonal encasement may be considered as a suitable protective structure for pipelines across alluvial rivers, with relatively modest effects on the local hydrodynamic conditions and bed stabilization.
Highlights
Subsurface pipeline crossings of open channels are widely used in gas transmission and water transfer engineering
The water level profiles were measured along the left and the right banks near the pipe encasement site
We focused on the flow energy transformation for the annual discharge and once-in-threeyear flood conditions, as river bed scouring downstream of the pipe encasement mainly occurs under relatively small discharges
Summary
Subsurface pipeline crossings of open channels are widely used in gas transmission and water transfer engineering. When crossing an alluvial river, the pipeline is theoretically required to be installed below scour depth, and adequate cover must be maintained under the bed [1,2]. The pipes buried initially might become exposed as a result of long-term river bed degradation, which increases the risk of pipeline damage caused by currents, waves, vibration, and human activities [4,5,6]. The encasement method, where pipes are completely encased in a rigid jacket (e.g., reinforced concrete), has increasingly been used to protect the buried pipelines from unpredictable loads and risks. Concrete encasement is the most effective and economical means of preventing pipeline failures from deterioration, corrosion or mechanical damage.
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